Re: Pivoting parts of a database table into an HTML table

[Anonymous Monk]

I suppose I need to post my half-solution, too. (My original post was getting long.) It's supposed to replace the rather common looping code:

my $lastid = 0;
while ($sth->fetch) {
    print $row;
    if ($lastid != $id) {
        print "</tr><tr>";
    }
}

with this:

alternate(
    sub { $sth->fetch },
    sub { print $row },
    sub { $id },              # which variable to monitor
    sub { print "</tr><tr>" } # what to do when variable changes
);

Currently it has at least an off-by-one error (the words go into the wrong columns), but I can't spot the error.

sub alternate {
    my $iter = shift;
    my $loop = shift; # main loop callback
    my @conds;
    while (@_ > 0) {
        # sub to get value, sub to call if value changed
        push @conds, [ shift, shift ];
    }

    my @memory;
    return unless $iter->();
    # starting values
    @memory = map { $_->[0]->() } @conds;

    do {
        $loop->();

        while (my ($idx, $cond) = each @conds) {
            my $new = $cond->[0]->();

            next if $memory[$idx] eq $new;

            $cond->[1]->(); # callback

            $memory[$idx] = $new;
            # reset subsequent values
            $memory[$_] = $conds[$_][0]->() for ($idx + 1 .. $#conds);

            last;
        }
    } while ($iter->());
}

my $gen = make_generator(\my ($day, $type, $word));

alternate(
    $gen,
    sub { print "$word, " },
    sub { $day },
    sub { print "</td></tr>\n<tr><td>$day</td><td>" },
    sub { $type },
    sub { print "</td><td>" }
);